Critics say two of the district's three elementary schools, Wayne and Ithan, are crowded and for years have relied on modular classrooms that were supposed to be temporary.
"Adding students in an overcrowded building is going to further reduce the quality of life of children while they're at school," said Ylva Kelsall, who helped start the petition after two December school board meetings to outline the plan, and who has a third grader at Wayne and two children starting kindergarten next fall.
She and other Wayne parents say there are not always enough rooms for art and music, lunches start at midmorning, and parents have been told club activities such as chess and Girl Scouts would be bumped back to make room for after-school care for kindergartners.
"It's quite shocking for a township like Radnor to have students playing instruments in the hallways, having art delivered on a cart, and eating lunch at 10:30," Kelsall said.
At Ithan, the entire fifth grade class would be moved to a makeshift building to make room for the kindergartners.
District administrators have indicated that the board is unlikely to reconsider the plan, which was first approved by the school board three years ago but whose implementation has already been delayed once.
In a 2012 district survey, nearly 400 respondents said they would support a move to full-day kindergarten. And officials say the inconvenience will be temporary and they plan to expand both schools and eliminate the trailers in time.
"It may not be an ideal situation for a year or two, but we are willing to accept that trade-off because we believe the trade-off is worth it," Board President Eric Zajac said.
Until now, Radnor has been at odds with trends elsewhere in the state. During the 2000s, the Delaware County district stuck with its half-day program when many other districts - even economically challenged ones such as Philadelphia - were moving to full-day classes.
Kindergarten is not mandatory in Pennsylvania. But more than 450 out of 501 districts offer full-day kindergarten, according to the state Department of Education. In about 375, it is the only option.
The planned expansion comes at a time when some Pennsylvania districts have eliminated their full-day programs because of funding woes.
In Radnor, money is not an issue - district officials say they won't need a tax increase to cover the $2.8 million it will cost to offer a full-day program for 250 students, an increase of about 38 from this year.
But that has done little to mollify the parents who say the move will clog cafeterias and traffic loops, and create safety issues as children move to and from the trailers.
Julia Bohnenberger, who has a second grader at Wayne, said the expanded program will impact other already squeezed students, and raises safety issues for students who use the trailers and have to leave their classrooms to go outside to use bathrooms.
"All this talk about safety. Oh, we're putting buzzers on the front doors, that's great. But kids can get locked out of their classrooms and no one knows they're in the bathroom," she said.
According to a district report, students and staff will be crammed. With kindergartners gobbling up classroom space, plans call for some children to eat lunch in their classrooms; use art, music, gym, and computer carts to deliver supplies to students rather than have designated space; and send some of the overflow at Radnor Elementary to another school.
Wait times for student pickup and drop-off will be longer, and some students will have longer bus trips, the report said.
Superintendent Michael Kelly said those were only possible outcomes but acknowledged "there will be some adjustments."
Zajac said he considers expanded kindergarten necessary for students to meet the new state-mandated Common Core standards and compete in a global economy.
"There are parts of the world where students spend a lot more time in classrooms than our students," he said.
There may be a cultural issue involved, but one entirely different from what Zajac was talking about. In Radnor, where many mothers can afford to not work, some would prefer to have their children at their side for at least part of the day.
Kelsall, 44, said she retired from her job in pharmaceutical manufacturing when her twins were born and would like them home another year.
"If [full-day kindergarten] is going to be implemented, it should be implemented with the acknowledgment that not all children are ready for a full day," she said. "I enjoy being home with my children, doing things in the afternoon with them. And I think a 1-2 ratio is much more nourishing than a 1-20 ratio."